In industry, the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of different products of varying shape and thickness are being produced, and different quality requirements are placed on these products depending on their final use. In order to meet the different requirements the powder metallurgy industry has developed a wide variety of iron and iron-based powder compositions.
One processing technique for producing the parts from these powder compositions is to charge the powder composition into a die cavity and compact the composition under high pressure. The resultant green part is then removed from the die cavity. To avoid excessive wear on the die cavity, lubricants are commonly used during the compaction process. Lubrication is generally accomplished by blending a solid, particular lubricant powder with the iron-based powder (internal lubrication) or by spraying a liquid dispersion or solution of the lubricant onto the die cavity surface (external lubrication). In some cases, both lubrication techniques are utilized.
Lubrication by means of blending a solid lubricant into the iron-based powder composition is widely used and new solid lubricants are developed continuously. These solid lubricants generally have a density of about 1-2 g/cm3, which is very low in comparison with the density of the iron-based powder, which is about 7-8 g/cm3. Additionally, in practice the solid lubricants have to be used in amounts of at least 0.6% by weight of the powder composition. As a consequence the inclusion of these less dense lubricants in the composition lowers the green density of the compacted part.
Liquid lubricants in combination with iron powders for the preparation of compacted parts are disclosed in the U.S. Pat. No. 3,728,110. According to this patent it is necessary to use the lubricant in combination with a particulate porous oxide gel. Furthermore, the examples of this patent disclose that also a conventional solid lubricant (zinc stearate) is used. The iron powder tested was an electrolytic powder having a particle size less than 80 mesh (US Standard Sieve size). Also the U.S. Pat. No. 4,002,474 concerns liquid lubricants. According to this patent discrete pressure-rupturable microcapsules are used. The microcapsules comprise a core and a solid shell surrounding the core, which includes an organic liquid lubricant. In the type of lubricant system disclosed in the U.S. Pat. No. 6,679,935 a lubricant, which is solid at ambient conditions, melts upon application of pressure during the pressing of the metal parts and the lubricant system forms a liquid phase along the walls of cavity, in which the powder is being pressed. In modern PM technology, however, liquid lubricants per se have not been successful.
It has now unexpectedly been found that when iron or iron based powders of a certain type are combined with a specific type of liquid organic substances as lubricants, it will be possible to obtain compacted bodies having not only high density but it has also been found that these compacted bodies can be ejected from the dies with comparatively low ejection forces. Furthermore it has turned out that these lubricants are effective in preventing wearing of the walls of the die and the surfaces of the compacted bodies are without remarks. In contrast to the teaching in the U.S. Pat. No. 3,728,110 particulate no porous oxide gel is needed.